This article proposes a new way to improve the protein quality Of the common bean (Phaseolus vulgaris). it is based on the natural variability found in the different types of phaseolin, its main storage protein (40-50% of the total protein). Despite the fact that it is deficient in methionine content, phaseolin still represents the main source of that amino acid in the seed. More than 40 genetic variants, differing in subunit number (2-6) and molecular weight (40-54 kDa) have been analyzed. The similarity of the amino acid composition among phaseolins, suggests that a nutritional improvement cannot be expected from that side. Conversely, important variation in phaseolin Susceptibility to proteolysis (ranging from 57% to 96% after cooking) has been observed, increasing the theoretical availability of methionine by up to 37%. Therefore, breeding programs based oil highly-digestible phaseolin types could lead to the production of beans with higher protein quality. (C) 2009 Published by Elsevier Ltd.